The present invention relates generally to piezoelectric sensors and actuators and, more particularly, to piezoelectric laminates, segmented piezoelectric film sensors, and the application thereof to modal analysis.
During the past decade, distributed piezoelectric sensors and actuators (collectively, transducers) have been used increasingly in the field of active vibration sensing and control. The basic idea is to bond a piezoelectric lamina onto a large surface of a thin vibrating structure. By collecting the piezoelectric charge induced by surface strains, one can estimate the vibrational state of the structure. Reversely or conversely, by applying an appropriate control voltage to the transducer, it is possible to excite (actuate) the structure and thus, for example, to generate an artificial damping effect or monitor structural behavior. Among the advantages of this approach are the light weight of the piezoelectric transducer and the possibility to embed the device in thin, flexible structures that are prone to vibrate. Use of this technology can be found from the aerospace industry to manufacturers of high-end consumer goods such as xe2x80x9csmart skisxe2x80x9d (i.e. skis that incorporate such transducer technology).
For this technology to be efficient, it is necessary for the polarization of the piezoelectric lamina to vary in a well-defined manner over the surface of the transducer. Since the piezoelectric sensitivity of a laminate is normally uniform, a weighting function must be applied to the transducer with the help of some controlled physical process.
It has been demonstrated that by measuring or inducing strains in a thin bending structure (such as a beam or plate) by means of a shaped piezoelectric lamina bonded on its surface, it is possible to sense or actuate directly a given modal contribution of a transverse motion, provided that the boundary conditions satisfy certain requirements that result in the orthogonality of these modes.
In this method, it can be shown that the local piezoelectric sensitivity of the transducer must follow an appropriate spatial sensing distribution (or weighting) function xcex9(xcex11, xcex12) in order to interact with a unique mode. It should be noted that the use of the two parameters xcex11 and xcex12 do not limit the domain of xcex9 to a plane. Instead, xcex11 and xcex12 are bi-dimensional curvilinear coordinates that can define any point on the neutral surface of a three-dimensional shell. Since the piezoelectric sensitivity of a laminate is usually uniform, a weighting function must be implemented with the help of a controlled physical process. Sometimes, the operation of imposing a given polarization profile on a planar piezoelectric transducer is also called xe2x80x9cshadingxe2x80x9d. There are several available procedures for shading.
In one procedure, the longitudinal piezoelectric constants of the piezoelectric material can be adjusted to a value proportional to a weight function xcex9(xcex11, xcex12), by means of a manufacturing method such as repoling, doping or the dosage of a two-phase composite. It should be noted however that these techniques are costly and difficult to implement in practice.
In another procedure, a given area of a piezoelectric transducer is made effective by the presence of a pair of electrodes which can either collect the charges generated piezoelectrically (sensor mode) or impose an electric potential between them (actuator mode). Thus, an elementary type of spatial weighting is obtained by limiting the areas provided with electrodes. In this case, the weighting function takes only two values: xcex9=1 inside the shape boundaries, or xcex9=0. In practice, a photolithographic process can implement this profile.
For a beam, it is sufficient to implement a one-dimensional weight function. In this case, the appropriate variation of piezoelectric sensitivity can be obtained by varying the width of the electrode down the length of the beam since there is no deflection along this direction. To impose the correct sign of the weight (spatial sensing distribution) function, areas are defined where the polarization of the piezoelectric material should be accordingly positive or negative, either by bonding the laminate xe2x80x9cface upxe2x80x9d or xe2x80x9cface downxe2x80x9d, or by inverting its contacting electrodes.
In the more general case of a structure whose deflection varies along two directions, a bi-dimensional spatial sensing distribution function can be approximated by a lattice of small electrodes that are turned either xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d. However the task remains to connect electrically the individual active electrodes and to impose the correct function sign. In practice, this is only feasible if the xe2x80x9conxe2x80x9d electrodes happen to be grouped in a few contiguous domains.
In spite of the elegance of their concept, modal sensors and actuators have an important limitation. A manufacturing process allowing control the weight function by repoling, doping, or dosing a two-phase composite is usually not available at the level of the application engineer. Furthermore, these steps would be very costly to implement. As a consequence modal sensors have not been applied to structural elements, such as plates and shells, unless their weight function could be reduced to a one-dimensional function or a product of such functions by separation of variables.
This difficulty and other factors have promoted the use of an approximated version of modal sensors, called segmented piezoelectric sensors. In this design the distributed effect of a piezoelectric laminate is replaced by an array of size-limited, discrete piezoelectric sensors, each of which are measured separately, and the outputs of which are being sampled, multiplied by discrete weight factors (calculated by the method of modal filtering), and then added. The main advantage of this method is that it shifts the operation of fixing the weight factors from the manufacturing process to an electronic operation, making it much more flexible. Thus, except for the number of channels, it is not more difficult to build such a system for a variety of shells and plates rather than a beam.
However, segmented piezoelectric sensors also have their shortcomings. For one, segmented piezoelectric sensors are only able to model a finite number of modes (the more transducers in the array, the higher this number). If unmodelled modal contributions (residual modes) are present, they constitute a source of noise. For another, segmented piezoelectric sensors are much less compact than modal sensors. Each channel requires a full measurement chain including coaxial cable, low current or charge sensitive amplifier and analog to digital converter. Furthermore, a digital signal processing board is required to carry out the computations to estimate the modal coordinates. By contrast, a modal sensor simply needs a unique coaxial cable and a low current or charge sensitive amplifier. For this reason it is more difficult to embed segmented piezoelectric sensors as elements of an intelligent structure. Finally, because of the analog to digital conversions involved with segmented piezoelectric sensors, it is difficult to include segmented transducers in sensing-actuating applications, such as the frequency-stabilizing element in a resonator. On the other hand, the use of a modal sensor in a resonating circuit is straightforward and may open the door to applications where the frequency of such a system could be used to monitor physical parameters, like the temperature of the structure or a variation of pressure exercised on it.
Because of the limitations imposed by these technologies, it is very difficult to obtain a polarization profile whose shape varies in function of two geometric dimensions. Thus, in order to apply this active control scheme to structures whose deflections vary along two independent coordinates, such as vibrating plates and shells, one of the following simplifying methods is normally used.
One such method is that the behavior of a bending plate may be approximated with the one of a bending beam, so that the electrode shaping method described above may be used.
Another such method is that an arbitrary, bi-dimensional polarization profile may be approximated with a discrete pattern. Namely, one may juxtapose a finite set of segmented piezoelectric sensors/actuators whose individual contributions are weighted and added up electronically.
A further such method is that a polarization profile can be roughly approximated by applying a xe2x80x9cbinaryxe2x80x9d weighting function, i.e. one in which contiguous areas are multiplied by values of either 0 or 1. This can be realized by bi-dimensional shaping of the measuring electrodes.
However, in each of these simplified embodiments, the efficiency of the control scheme is eventually affected. Possible problems may range from perturbations by residual modes to instabilities via spillover. The use of segmented sensors/actuators may also pose practical problems if many channels are required because of extra weight added to the test structure by the connect cables. As a consequence, there is a need for a new generation of distributed piezoelectric transducers in which arbitrary polarization profiles could be implemented in a versatile and practical manner.
There is also a need for a piezoelectric transducer that can efficiently implement a weighting function/distribution.
There is further a need for a new generation of segmented piezoelectric transducers.
In one form (referred to herein as Form A), the present invention is a piezoelectric transducer. Particularly, in one form the present invention is a multi-layered piezoelectrically active and inactive lamina structure having selective metalizations that implement a bi-dimensional weighting function of piezoelectric sensitivity.
When the lamina structure is attached to a thin structural element, the piezoelectric transducer may be used as a modal sensor and/or actuator.
The present invention provides at least the following advantages over known solutions to the problem of sensing or actuating transverse modes of vibration in thin structures, which may be applied to various applications too numerous to list: (1) compared to shaped piezoelectric modal sensors and actuators, it is straightforward to implement with this design any arbitrary two-dimensional weight function for structure such as plates and shells. Also, the problem of forcing the correct sign of the weight function is solved in the invention by collecting piezoelectrically induced charges on both faces/surfaces of active lamina of the piezoelectric laminate in a first configuration, and on two individual laminas of the piezoelectric laminate in a second configuration; (2) modal sensors whose weight functions have been built-in by modulating the piezoelectric sensitivity of the material by a manufacturing means, such as repoling, are very costly and often the technology is not readily available, if at all, to engineers, while the technologies required by the present invention in order to implement the weight function (e.g. photolithography or screen printing) are widely available and economical; (3) segmented piezoelectric sensors require a full measurement chain (i.e. a transmitting cable, an impedance converter such as a charge amplifier, and a sampling circuit converting the analog signal into digital form) for each channel. This prior approach becomes very expensive, and thus not practical for modal filtering because of the high number of channels. As well, perturbations by residual modes are then more likely to occur. By contrast, a very high density of weighted xe2x80x9cchannelsxe2x80x9d can be implemented with the present invention at no special cost, the limitation being set by the resolution of the available photolithographic or screen-printing process. Also, prior segmented piezoelectric sensors and actuators require an analog to digital conversion (digital to analog, respectively) and dedicated digital signal processing boards. For the present invention, all that is needed is a low-current or charge sensitive amplifier in the sensing mode and a sine voltage generator in the actuating mode for the present piezoelectric laminate.
In another form (herein referred to as Form B), the present invention is a segmented piezoelectric film sensor. The segmented piezoelectric film sensor includes a set of active elements mounted onto a base and connected with external circuitry. The set of active elements includes piezofilms/sensors, individual electronics for each segment, and a flexible printed circuit. The external circuitry provides a modulation scheme to combine individual sensor outputs and to extract modal coordinates from a test structure on which the segmented piezoelectric sensor is mounted.
The invention has advantages over prior segmented piezoelectric sensors and to modal filtering using segmented piezoelectric sensors. In particular, compared to prior segmented piezoelectric sensors, the present invention simplifies the required hardware by replacing N-2 measurement chainsxe2x80x94each comprising a low current or charge sensitive amplifier and a coaxial cablexe2x80x94with 2N digital modulation signals and electric wires, and by eliminating the need for a digital signal processing board, the system output being a signal directly proportional to the modal coordinate of interest. Also, the invention is always able to provide an estimate of the modal coordinate in real-time, even if a very large number of piezosensors have to be taken into account. Finally, the use of a flexible printed circuit in the present invention makes it easier to connect the active components mounted on the structure to the external electronic circuitry. As well, compared to shaped piezoelectric modal sensors, the system has the flexibility of being able to modify its weighting function, either to improve the estimate of the modal coordinate of interest, or to monitor a different modal coordinate.